If you want to enable FaceTime on your iPhone 3GS, you can do so, it's not really a most wanted feature yet having FaceTime in your iDevice will give excitement. Anyway, you can follow the instructions to enable FaceTime for iPhone 3GS on iOS 4.1 here.

Requirement: Your iPhone must already been jailbreak because the instruction requires you to download an app from Cydia, named FaceIT-3GS.